(a) Field of the Invention
This invention relates to diagnostic radiology and in particular relates to the use of high energy radiation to form images of internal structures upon a sensing means such as a luminescent, e.g., fluorescent screen. The invention more particularly relates to a method and apparatus for reducing detection of radiation scatter in forming such images.
(b) History of the Prior Art
Originally, X-ray photographs were taken simply by directing X-rays from a source, e.g., an original roentgen ray tube, through an object, such as an anatomical structure, to a detector such as an X-ray film. This initial historical arrangement did not include additional devices to reduce hazards to a patient or to enhance the quality of the detected X-ray image.
Later X-ray devices not only used improved X-ray tubes, such as tubes which in conjunction with other components could control intensity and wave length of X-rays, but also incorporated filters for eliminating radiation outside of the useful X-ray range and included means for enhancing the contrast of the image by eliminating at least a portion of radiation scatter from the object, such as a patient, to the detector. Such scatter often results when high energy radiation interacts with atomic fields or particles. Scattered radiation is usually emitted in a direction different than the direction of the incoming primary radiation. The scattered radiation thus causes exposure of the detector to radiation at all locations thus reducing contrast of the detected image with the background.
The most usual method for reducing scatter historically has been and remains radiographic grids which consist of a series of lead foil strips separated by X-ray partially transparent spacers. The lead foil acts to intercept secondary or scattered radiation which approaches the detector at an inappropriate angle. Such grids have, however, certain undesirable characteristics. For example, contrast is still not as high as desired since all scatter is still not eliminated, primary radiation is absorbed by interspaces and grid lines become apparent on the X-ray image since the lead strips absorb primary radiation from the radiation source.
Attempts have been made to eliminate the appearance of the grid pattern in the X-ray image by moving the grid in a direction essentially perpendicular to radiation passing from the source to the detector; however, such a moving grid did nothing to increase contrast. In addition, since the angle of radiation from a fixed source to a point on a planar grid changes as the grid moves, an appropriate fixed angle between grid members to allow maximum passage of primary radiation between the members is not possible. There has also been an attempt to utilize linearly moving aligned slit devices to increase contrast. The devices have to be moved at a uniform speed and have to reach that speed before the X-ray is taken. This requires that the X-ray be taken in a precisely timed relationship to slit motion. Furthermore, vibration within the system cannot be tolerated and uniform X-ray output must be maintained during exposure to prevent unwanted artifact patterns on the detector. The moving slit system is also subject to the problem of radiation angle change as the slits move thus requiring complex mechanical devices to change the angle of the slits as they move.
Attempts have been made to use rotating planar disks to reduce the detection of secondary radiation. See French Pat. No. 521,746 (1921); Rudin et al., "Rotating Aperature Devices in Conventional Tomography," Proceedings of the Society of Photo-Optical Instrumentation Engineers (SPIE) Vol. 233, (1980); Rudin, "Fore-and-Aft Rotating Aperature Wheel (RAW) Device for Improving Radiographic Contrast," SPIE, Vol. 173, (1979); and Sorenson et al., "Rotating Disc Device for Slit Radiography of the Chest," Radiology, Vol. 134, (January 1980).
All of these disk devices have major deficiencies. In particular unless the disk or disks rotate on the same perpendicular axis upon which the radiation source is located, the problem of radiation angle change as the holes or slits move is not alleviated and if the disks do rotate on the same perpendicular axis upon which the radiation source is located, X-rays may not be used perpendicularly to the plane of the disk since perpendicular rays pass through the central axis which contains the means for supporting the disk. The inability to use rays perpendicular to the disk means that the disks may not be parallel to the object being X-rayed unless distorted oblique X-ray views are desired. When a disk is placed at an angle to the object, large undesirable distances between the object and detector are required.
Devices using concentric rotating cylinders have also been described. See e.g., French Pat. No. 521,746 which describes concentric moving bands and Rudin, "Rotating Aperature Devices in Conventional Tomography," SPIE, Vol. 233, (1980), which describes cylinders for use in conjunction with Computed Tomography (CT) devices. Concentric rotating cylindrical devices are undesirably large since the object must be placed within any cylinders having walls between the object and detector